Fissile material Although sometimes used as a synonym
for fissionable material, this term has acquired a more restricted meaning.
Namely, any material fissionable by thermal (slow) neutrons. The
three primary fissile materials are uranium-233, uranium-235, and plutonium-239.

Uranium

A radioactive element with the atomic number 92
and, as found in natural ores, an atomic weight of approximately 238.
The two principal natural isotopes are uranium-235 (0.7 percent
of natural uranium), which is fissile, and uranium-238 (99.3 percent of
natural uranium), which is fissionable by fast neutrons and is
fertile. Natural uranium also includes a minute amount of uranium-234.

U235 and fission

The uranium atom was actually split into two atoms
of approximately the same size -- and fission was accomplished. This released
significant amounts of energy. It was found that an isotope of uranium,
uranium-235, easily fissioned with slow neutrons to yield krypton and
barium. (Taylor, 353) Unfortunately,
uranium-235 is found in naturally occurring uranium only about 1 part
in 137. Extracting it is not an easy process. (Segre, "From X-rays...",

The element with the most protons known at the
time was uranium. It has 92 protons and 146 neutrons. (It is usually known
as uranium-238.)

Isotope

One of two or more atoms with the same number
of protons, but different numbers of neutrons in their nuclei.
Thus, carbon-12, carbon-13, and carbon-14 are isotopes of the element
carbon, the numbers denoting the approximate atomic weights. Isotopes
have very nearly the same chemical properties, but often different physical
properties (for example, carbon-12 and -13 are stable, carbon-14 is radioactive).

Plutonium

Plutonium has assumed the position of dominant
importance among the trasuranium elements because of its successful use
as an explosive ingredient in nuclear weapons and the place which it holds
as a key material in the development of industrial use of nuclear power.
One kilogram is equivalent to about 22 million kilowatt hours of heat
energy. The complete detonation of a kilogram of plutonium produces an
explosion equal to about 20,000 tons of chemical explosive. Its importance
depends on the nuclear property of being readily fissionable with neutrons
and its availability in quantity. The world's nuclear-power reactors are
now producing about 20,000 kg of plutonium/yr. By 1982 it was estimated
that about 300,000 kg had accumulated. The various nuclear applications
of plutonium are well known. 238Pu has been used in the Apollo lunar missions
to power seismic and other equipment on the lunar surface. As with neptunium
and uranium, plutonium metal can be prepared by reduction of the trifluoride
with alkaline-earth metals.

A reactor in which the fission chain reactionis
sustained primarily by fast neutrons rather than by slow moving
neutrons. Fast reactors contain little or no moderator to slow
down the neutrons from the speeds at which they are ejected from fissioning
nuclei.

Moderator

A material, such as ordinary water, heavy water,
or graphite, that is used in a reactor to slow down high-velocity
neutrons, thus increasing the likelihood
of fission.

Heavy Water

Water containing significantly more than the natural
proportions (one in 6,500) of heavy hydrogen (deuterium, D) atoms to ordinary
hydrogen atoms. Heavy water is used as a moderator in some reactors because
it slows down neutrons effectively and also has a low probability of absorption
of neutrons.

According to the Copenhagen interpretation it is
neither possible nor reasonable to search for properties of a quantum
system as such. Since we can only communicate what we have found by using
our classical language, questions concerning properties of systems only
make sense, strictly speaking, as questions about classical properties
of a classical apparatus.
See On the Interpretation and Philosophical Foundation of Quantum MechanicsAnton Zeilinger: http://www.uibk.ac.at/c/c7/c704/qo/philosop.html

Definition: A thought experiment introduced
by Erwin Schrödinger in 1935 to illustrate the paradox in quantum mechanics
regarding the probability of finding, say, a subatomic particle at a specific
point in space. According to Niels Bohr, the position of such a particle
remains indeterminate until it has been observed. Schrödinger postulated
a sealed vessel containing a live cat and a device triggered by a quantum
event such as the radioactive decay of the nucleus. If the quantum event
occurs, cyanide is released and the cat dies; if the event does not occur
the cat lives. Schrödinger argued that Bohr's interpretation of events
in quantum mechanics means that the cat could only be said to be alive
or dead when the vessel has been opened and the situation inside it has
been observed. This paradox has been extensively discussed since its introduction
with many proposals made to resolve it.

"Exactly what would happen, they asked themselves,
if you could cull from natural uranium a mass composed purely of the rare
uranium-235? Bohr and others had told the public that there could be enough
energy there to blow up a city, but nobody had worked it out as a serious
technical possibility. Now Frisch and Peierls realized that with fissionable
uranium-235 atoms all crammed together, there would be no need for a moderator
to slow the neutrons down, since even the fast neutrons emitted in each
fission would have a good chance to provoke another fission. The whole
chain reaction would go so swiftly that, before the mass had a chance
to blow itself apart, a run away [reaction would allow] many of the uranium-235
atoms [to] split and release energy." (Weart,
84)

This question may have been left academic for years
had it not been for World War II. As the awesome power of an atomic bomb
was realized by leaders of several countries, a race began to be the first
to make a working bomb. As a result, a simpler method was discovered than
separating uranium-235 from uranium-238.

This simpler method starts when uranium-238 absorbs
a single neutron a new element, called neptunium-239, is created. (Neptunium-239
has 93 protons and 146 neutrons.) This element decays into plutonium-239
(94 protons and 145 neutrons). Plutonium is stable and also has the property
of undergoing fission with slow neutrons. Hence, the atom bomb was conceivable.
Plutonium was produced in a reactor. (Weart,
87)

ATOM BOMB
is a weapon with great explosive power that results from the sudden release
of energy upon the splitting, or fission,
of the nuclei of such heavy elements as plutonium or uranium.

When a neutron strikes the nucleus of an atom of
the isotopes uranium 235 or plutonium-239,
it causes that nucleus to split into two fragments, each of which is a
nucleus with about half the protons and neutrons of the original nucleus.
In the process of splitting, a great amount of thermal energy,
as well as gamma rays and two or more neutrons, is released. Under
certain conditions, the escaping neutrons strike and thus fission more
of the surrounding uranium nuclei, which then emit more neutrons that
split still more nuclei. This series of rapidly multiplying fissions culminates
in a chain reaction in which nearly all the fissionable material is consumed,
in the process generating the explosion of what is known as an atomic
bomb.

Many isotopes of uranium can undergo fission, but
uranium-235, which is found naturally at a ratio of about one part per
every 139 parts of the isotope uranium-238, undergoes fission more readily
and emits more neutrons per fission than other such isotopes. Plutonium-239
has these same qualities. These are the primary fissionable materials
used in atomic bombs. A small amount of uranium-235, say 0.45 kg (1 pound),
cannot undergo a chain reaction and is thus termed a subcritical mass;
this is because, on average, the neutrons released by a fission are likely
to leave the assembly without striking another nucleus and causing it
to fission. If more uranium-235 is added to the assemblage, the chances
that one of the released neutrons will cause another fission are increased,
since the escaping neutrons must traverse more uranium nuclei and the
chances are greater that one of them will bump into another nucleus and
split it. At the point at which one of the neutrons produced by a fission
will on average create another fission, critical
mass has been achieved, and a chain reaction and thus an atomic explosion
will result.

In practice, an assembly of fissionable material
must be brought from a subcritical to a critical state extremely suddenly.
One way this can be done is to bring two subcritical masses together,
at which point their combined mass becomes a critical one. This can be
practically achieved by using high explosives to shoot two subcritical
slugs of fissionable material together in a hollow tube. A second method
used is that of implosion, in which
a core of fissionable material is suddenly compressed into a smaller size
and thus a greater density; because it is denser, the nuclei are more
tightly packed and the chances of an emitted neutron's striking a nucleus
are increased. The core of an implosion-type atomic bomb consists of a
sphere or a series of concentric shells of fissionable material surrounded
by a jacket of high explosives, which, being simultaneously detonated,
implode the fissionable material under enormous pressures into a denser
mass that immediately achieves criticality. An important aid in achieving
criticality is the use of a tamper; this is a jacket of beryllium oxide
or some other substance surrounding the fissionable material and reflecting
some of the escaping neutrons back into the fissionable material, where
they can thus cause more fissions. In addition, "boosted fission"
devices incorporate such fusionable materials as deuterium or tritium
into the fission core. The fusionable material boosts the fission explosion
by supplying a superabundance of neutrons.

Fission releases an enormous amount of energy relative
to the material involved. When completely fissioned, 1 kg (2.2 pounds)
of uranium-235 releases the energy equivalently produced by 17,000 tons,
or 17 kilotons, of TNT. The detonation of an atomic bomb releases enormous
amounts of thermal energy, or heat, achieving temperatures of several
million degrees in the exploding bomb itself. This thermal energy creates
a large fireball, the heat of which can ignite ground fires that can incinerate
an entire small city. Convection currents created by the explosion suck
dust and other ground materials up into the fireball, creating the characteristic
mushroom-shaped cloud of an atomic explosion. The detonation also immediately
produces a strong shock wave that propagates outward from the blast to
distances of several miles, gradually losing its force along the way.
Such a blast wave can destroy buildings for several miles from the location
of the burst. Large quantities of neutrons and gamma rays are also emitted;
this lethal radiation decreases rapidly over 1.5 to 3 km (1 to 2 miles)
from the burst. Materials vaporized in the fireball condense to fine particles,
and this radioactive debris, referred to as fallout,
is carried by the winds in the troposphere or stratosphere. Since the
radioactive contaminants include such long-lived radioisotopes as strontium-90
and plutonium-239, they can have lethal effects for weeks after the explosion.

The first atomic bombs were built in the United
States during World War II under a program called the Manhattan
Project. One bomb, using plutonium, was successfully tested on July
16, 1945, at a site 193 km (120 miles) south of Albuquerque, N.M. (seephotograph). The first atomic bomb to be used in warfare used uranium.
It was dropped by the United States on Hiroshima, Japan, on Aug.
6, 1945. The explosion, which had the force of more than 15,000 tons of
TNT, instantly and completely devastated 10 square km (4 square miles)
of the heart of this city of 343,000 inhabitants. Of this number, 66,000
were killed immediately and 69,000 were injured; more than 67 percent
of the city's structures were destroyed or damaged. The next atomic bomb
to be exploded was of the plutonium type; it was dropped on Nagasaki
on Aug. 9, 1945, producing a blast equal to 21,000 tons of TNT. The terrain
and smaller size of Nagasaki reduced destruction of life and property,
but 39,000 persons were killed and 25,000 injured; about 40 percent of
the city's structures were destroyed or seriously damaged. The Japanese
initiated surrender negotiations the next day.

After the war, the United States conducted dozens
of test explosions of atomic bombs in the Pacific at Enewetak
(Eniwetok) atoll and in Nevada. In subsequent years, the Soviet Union
(1949), Great Britain (1952), France (1960), China (1964), India (1974),
and Pakistan (1998) tested fission weapons of their own. The great temperatures
and pressures created by a fission explosion are also used to initiate
fusion and thus detonate a thermonuclear bomb..

On the 2nd August 1939 some scientists wrote to
President Roosevelt of efforts in Nazi Germany to purify Uranium-23 5
with which might in turn be used to build an atomic bomb. It was shortly
thereafter that the United StatesGovernment began the serious undertaking
known only then as the Manhattan Project. The Manhattan Project was designed
to research and production that would produce a usable atomic bomb. The
Project was named after the Manhattan Engineer District of the US Army
Corps of Engineers, because a lot of the early research was done in New
York.

In 1942 General Leslie Grove was chosen to lead
the project. He brought a site at Oak Ridge, Tenn. For facilities to separate
the necessary uranium-235 from the much more common uranium-238. Robert
Oppenheimer was appointed to lead the day to day running of the project.
The team of scientists who worked on the atom bomb worked 6 days a week
and often 18 hours a day.

By 1945 the project has nearly 40 laboratories
and factories which employed 200,00 people. That was more than the total
amount of people employed in the US automobile industry in 1945. The total
cost of the Manhattan project was $2-billion which is about the equivalent
of $26 billion today.

Historically, the Copenhagen interpretation was
the firstto recieve wide recognition (1927);
it basically asserts that experimentalpredictions
are the reality, and that attempts to model underlyingmechanisms
are futile. There is virtually no agreement on the details,even
among advocates. Today, it is widely seen ....as
only one among the possible interpretations.Most
other interpretations are ontological, which means theyassume
some underlying model for reality which, if not exactly right, isat least supposed to capture whatever features are
relevant of whatevermodel would be exactly
right.

(1)A real difficulty
in the understanding of this interpretation [of quantum theory] arises,
however, when one asks the famous question: But what happens 'really'
in an atomic event? It has been said before that the mechanism and the
results of an observation can always be stated in terms of the classical
concepts. But what one deduces from an observation is a probability function,
a mathematical expression that combines statements about possibilities
or tendencies with statements about our knowledge of facts So we cannot
completely objectify the result of an observation, we cannot describe
what 'happens' between this observation and the next. This looks as if
we had introduced an element of subjectivism into the theory, as if we
meant to say: what happens depends on our way of observing it or on the
fast that we observe it. Before discussing this problem of subjectivism
it is necessary to explain quite clearly why one would get into hopeless
difficulties if one tried to describe what happens between two consecutive
observations.

(2)To make this point
clearer we have to analyse the process of observation more closely.

To begin with, it is important to remember that
in natural science we are not interested in the universe as a whole, including
ourselves, but we direct our attention to some part of the universe and
make that the object of our studies. In atomic physics this part is usually
a very small object, an atomic particle or a group of such particles,
sometimes much larger - the size does not matter; but it is important
that a large part of the universe, including ourselves, does not belong
to the object.

Now, the theoretical interpretation of an experiment
starts with the two steps that have been discussed. In the first step
we have to describe the arrangement of the experiment, eventually combined
with a first observation, in terms of classical physics and translate
this description into a probability function. This probability function
follows the laws of quantum theory, and its change in the course of time,
which is continuous, can be calculated from the initial conditions; this
is the second step. The probability function combines objective and subjective
elements. It contains statements about possibilities or better tendencies
('potentia' in Aristotelian philosophy), and these statements are completely
objective, they do not depend on any observer; and it contains statements
about our knowledge of the system, which of course are subjective in so
far as they may be different for different observers. In ideal cases the
subjective element in the probability function may be practically negligible
as compared with the objective one. The physicists then speak of a 'pure
case'.

When we now come to 'the next observation. the
result of which should be predicted from the theory, it is very important
to realize that our object has to be in contact with the other part of-the
world, namely, the experimental arrangement, the measuring rod, etc.,
before or at least at the moment of observation. This means that the equation
of motion for the probability function does now contain the influence
of the interaction with the measuring device. This influence introduces
a new element of uncertainty, since the measuring device is necessarily
described in the terms of classical physics; such a description contains
all the uncertainties concerning the microscopic structure of the device
which we know from thermodynamics, and since the device is connected with
the rest of the world, it contains in fact the uncertainties of the microscopic
structure of the whole world. These uncertainties may be called objective
in so far as they are simply a consequence of the description in the terms
of classical physics and do not depend on any observer. They may be called
subjective in so far as they refer to our incomplete knowledge of the
world.

After this interaction has taken place, the probability
function contains the objective element of tendency and the subjective
element of incomplete knowledge, even if it has been a 'pure case' before.
It is for this reason that the result of the observation cannot generally
be predicted with certainty; what can be predicted is the probability
of a certain result of the observation, and this statement about the probability
can be checked by repeating the experiment many times. The probability
function does - unlike the common procedure in Newtonian mechanics - not
describe a certain event but, at least during the process of observation,
a whole ensemble of possible events.

(3)With regard to
this situation Bohr has emphasised that it is more realistic to state
that the division into the object and the lrest of the world is not arbitrary.
Our actual situation in research work in atomic physics is usually this:
we wish to understand a l certain phenomenon, we wish to recognise how
this phenomenon follows from the general laws of nature. Therefore that
part of matter or radiation which takes part in the phenomenon is the
natural 'object' in the theoretical treatment and should be separated
in this respect from the tools used to study the phenomenon. This again
emphasises a subjective element in the description of atomic events, since
the measuring device has been constructed by the observer, and we have
to remember that what we observe is not nature in itself but nature exposed
to our method of questioning. Our scientific work in physics consists
in asking questions about nature in the language that we possess and trying
to get an answer from experiment by the means that are at our disposal.
In this way quantum theory reminds us, as Bohr has put it, of the old
wisdom that when searching for harmony in life one must never forget that
in the drama of existence we are ourselves both players and spectators.
It is understandable that in our scientific relation to nature our own
activity becomes very important when we have to deal with parts of nature
into which we can penetrate only by using the most elaborate tools.